1. Field of the Invention
This invention relates to an infrared continuous zoom telescope or lens system and, more specifically, to the use of a diffractive surface to color correct an infrared continuous zoom afocal lens.
2. Brief Description of the Prior Art
It is known that different frequencies of light do not focus at the same location after passing through a lens system. For this reason, color correction has been built into prior art lens systems. The state of the art of infrared zoom lenses is set forth in an article of Allen Mann entitled "Infrared zoom lenses in the 1980s and beyond", Optical Engineering, May, 1992, Vol. 31, No. 5, pp. 1064-1071 and in a brochure of Lenzar ElectroOptics, Inc. entitled "Infrared 8:1 Zoom With Advanced Optical Technology". The Mann article generally describes two telescopes, the Pilkington telescope utilizing "Zulu" techniques for compactness and the Barr & Stroud zoom telescope. Both telescopes require an additional zinc selenide negative lens for color correction and require at least nine optical elements. The zinc selenide lens increases the cost, weight and complexity of the lens system. The Lenzar system uses diffractive aspheric optics in a zoom system in the 3 to 5 micron range but provides no structural details.